The invention relates to electrolyte systems for lithium batteries with enhanced safety, their use, and a method for enhancing the safety of lithium batteries.
Portable high-value electronic devices, such as mobile telephones, laptop computers, camcorders, etc. are enjoying a very fast growing market. An adequate electrical supply for these devices requires light, high-capacity and high-quality power sources. For environmental and economic reasons, secondary rechargeable batteries are overwhelmingly used. There are essentially three competing systems: nickel cadmium, nickel metal hydride, and lithium ion batteries. An additional interesting field of use for these battery systems could be their use in electrically operated vehicles.
Due to its outstanding performance characteristics, the lithium battery has already acquired large market shares, although it was introduced in the market in its current state of the art only in 1994. Despite the triumphant success of the secondary lithium battery, one cannot overlook the fact that it has safety-related problems:
Rechargeable lithium batteries typically contain a compound of lithium oxide and metal oxide as the cathode (e.g., LixMnO2 or LixCoO2) and lithium metal as the anode. The lithium is preferably used in the form of an intercalation compound with graphite or with carbon or graphite fibers. An overview of the use of such batteries is given by K. Brandt (Solid State Ionics 69 (1994), 173-183, Elsevier Science B. V.).
According to the current state of the art, the electrolyte liquids, which are used to achieve high conductivity, are preferably solvent mixtures of at least two or more components. The mixture must contain at least one strongly polar component, which due to its polarity has a highly dissociative effect on salts. The polar components that are typically used are ethylene carbonate or propylene carbonate. These highly polar solvents are relatively viscous and have usually a relatively high melting point, e.g., 35xc2x0 C. for ethylene carbonate.
To ensure adequate conductivity even at lower temperatures of use, one or more low-viscosity components are generally added as xe2x80x9cthinners.xe2x80x9d Typical thinners include, for instance, 1,2-dimethoxyethane, dimethyl carbonate ordiethyl carbonate. Usually the thinners are added in a proportion of 40-60% of the total volume. A serious drawback of these thinner components is their high volatility and their low flash point. 1,2-dimethoxyethane has a boiling point (BP) of 85xc2x0 C., a flash point (FP) of xe2x88x926xc2x0 C., and an explosion limit between 1.6 and 10.4% by volume; dimethyl carbonate has a BP of 90xc2x0 C., and an FP of 18xc2x0 C. For these xe2x80x9cthinnersxe2x80x9d there are currently no equivalent substitutes.
Since the electrochemical use of electrolyte solutions and, to a far greater extent, the occurrence of faults (short circuits, overcharging, etc.) always generates heat, this impliesxe2x80x94particularly if a cell bursts open and solvent spillsxe2x80x94a risk of ignition with the corresponding serious consequences. The currently used systems basically avoid this by using costly electronic controls. Nevertheless, some accidents caused by fire are known to have occurred, particularly during manufacture where large amounts of solvents are handled, but also during the use of rechargeable lithium batteries.
A greater source of risk during use is created in electrical vehicle applications. Here, substantially greater amounts of electrolyte liquid per energy storage device are required, and electronic control of many interconnected cells is far more difficult and involves correspondingly greater risks.
To enhance safety, the cathode and anode space can be separated by a microporous separator membrane, which is made in such a way that the current flow is interrupted by the melting of the pores when a certain temperature limit is exceeded. Suitable membranes of this type are found, for instance, in the Celgard(copyright) line of Hoechst Celanese Corporation.
The safety of lithium batteries can be further enhanced by pressure relief devices that respond to gas development if the battery is overcharged and, as mentioned above, by electronic monitoring and control devices.
Also recommended are flame-retardant additives containing phosphorus and halogen, but these often have a negative effect on the performance characteristics of the batteries.
All of these measures, however, cannot exclude the possibility that the highly volatile and flammable xe2x80x9cthinnerxe2x80x9d ultimately may be ignited in case of malfunctions and after rupture of the cell cause a fire that is difficult to control with common extinguishing agents. Burning lithium reacts violently not only with water but also with carbon dioxide, which is used in many fire extinguishers.
The following documents are cited as representative of the prior art:
JP-A-7 249432=D1
EP-A-0 631339=D2
EP-A-0 599534=D3
EP-A-0 575591=D4
U.S. Pat. No. 5,169,736=D5
B. Scrosati, ed., 2nd International Symposium on Polymer Electrolytes, Elsevier, London and New York (1990)=D6
U.S. Pat. No. 5,393,621=D7
JP-A-06020719=D8
U.S. Pat. No. 4,804,596=D9
U.S. Pat. No. 5,219,683=D10
JP-A-5 028822=D11, and
EP-A-0-821368=D12.
D1 and D2, for instance, propose highly fluorinated ethers as electrolyte solvents or as additives to other electrolytes. In general, these substances are thermally and chemically very stable and have high flash points. However their solvent power is far too low for the required lithium electrolyte salts, so that they cannot be used alone, and they are poorly miscible with conventional battery solvents.
Partially fluorinated carbonates are also described as electrolytes having an increased flash point (D3). The problem here is that the compounds, which apparently are suitable based on their low viscosity, have only a moderately increased flash point (37xc2x0 C.) and their electrical conductivities are clearly below those of the prior art (assuming that the reported measurements were taken at room temperature which seems likely since no temperature was specified).
Carbamates are also described as thinners for anhydrous electrolytes (D4). They have higher boiling points compared to the currently used thinners, but only marginally improved flash points.
D8 discloses ester compounds of the formula R1COOR2 as electrolytes for secondary lithium batteries, in which at least one of the groups R1 and R2 has a fluorine substitution. A preferred compound is trifluoroacetic acid methyl ester. However, this compound has a boiling point of only 43xc2x0 C. and a flash point of xe2x88x927xc2x0 C., which presents a high safety risk in case of damage.
According to the present state of the art, reduced flammability of the electrolyte solution is primarily achieved by increasing the viscosity of the electrolyte solution with the aid of binders or fillers or the use of polymer electrolytes, which are practically solid at room temperature.
D5, for instance, describes organic or inorganic thickeners (polyethylene oxide, SiO2, Al2O3 and others) for solidifying liquid electrolyte solutions.
Polymer electrolytes comprising macromolecules with numerous polar groups, such as polyethylene oxides, as they are known from D6, are also far less flammable due to their low volatility. One also frequently finds diacylated diols or monoacylated diol monoalkyl ethers as the monomer components for producing such a gel-like polymer electrolyte. In these substances the acyl component carries a double bond (i.e., it is, for example, an acrylic acid or methacrylic acid). Examples of such systems include references D11 and D12.
D7 describes polymer electrolytes comprising polar macromolecules formed by polymerization of organophosphorus compounds, which are characterized by their particularly low flammability.
All of these gel-like to solid electrolytes have in common that due to their high viscosity, the mobility of the ions of the salts dissolved in them is far lower than in liquid electrolyte solutions. As a result, particularly at lower temperatures, the conductivities required for most technical applications are no longer attained.
D9 claims esters, such as methyl formate and methyl acetate, as thinner components. From a safety aspect, however, these substances offer no advantages since they also have low flash points and boiling points.
D10 proposes diol diesters as electrolyte components and, among these, especially 1,2-diacetoxyethylene as the preferred substance. Although this substance has clear advantages with respect to its flash point compared to the typical thinners, its viscosity is so high that one of the conventional easily flammable thinners, such as dimethoxyethane, has to be added again to obtain the required conductivity.
Despite the efforts of the prior art, there has remained a need for improved electrolyte systems for secondary lithium batteries.
Accordingly, it was one object of the invention to provide novel electrolyte solvents for lithium batteries.
Another object of the invention was to provide electrolyte systems which are chemically and physically stable, adequately miscible with other suitable solvents, and adequately dissolve conductive lithium salts.
A further object of the invention was to provide electrolyte systems which have a clearly increased flash point, while nevertheless exhibiting a viscosity and conductivity behavior that makes them suitable in practical applications even at low temperatures.
In addition, in view of the increasing importance of rechargeable lithium cells, is was an object of the invention to provide an electrolyte system whose components would be recyclable in a simple manner.
These and other objects, which are not further defined but are readily apparent or can be derived from the introductory discussion of the prior art, are attained by an electrolyte system of the type described above which has the features described hereinafter.
Advantageous modifications of the electrolyte system according to the invention also are described as preferred embodiments.
The invention also includes a method for enhancing the safety of lithium batteries which provides a solution to the problems underlying the invention.
By providing an electrolyte system for lithium batteries with enhanced safety comprising at least one lithium-containing conductive salt and at least one electrolyte liquid, wherein the electrolyte liquid contains an effective amount of at least one partially fluorinated compound derived from a diol corresponding to formula (I)
R1COxe2x80x94Oxe2x80x94[CHR3(CH2)mxe2x80x94O]nxe2x80x94R2xe2x80x83xe2x80x83(I)
wherein
R1 is (C1-C8) alkyl or (C3-C8) cycloalkyl, wherein each of the aforementioned radicals is partially fluorinated or perfluorinated so that at least one hydrogen atom of the radical is replaced by fluorine,
R2 is (C1-C8) alkyl carbonyl or (C3-C8) cycloalkyl carbonyl, wherein each of the aforementioned radicals may optionally be partially fluorinated or perfluorinated,
R3 is hydrogen, (C1-C8) alkyl or (C3-C8) cycloalkyl,
m is 0, 1, 2 or 3, and
n is 1,2 or 3,
it is possible, in a particularly advantageous and not readily foreseeable manner to produce an electrolyte or an electrolyte system, which exceeds, or is at least equivalent to, the known electrolyte systems for lithium batteries within the usual requirement spectrum and at the same time provides increased safety compared to the prior art systems.
In particular, it has surprisingly been found that electrolyte systems for lithium batteries which include a compound of formula (I) meet the following spectrum of characteristics to an excellent degree:
high thermal stability
high flash point
low vapor pressure
high boiling point
low viscosity
miscibility with the usual solvents used for batteries, particularly with ethylene carbonate, propylene carbonate, diethyl carbonate or lactones, e.g., g-butyrolactone
adequate solvent power for fluorine-containing conductive lithium salts, e.g., LiPF6, LiN(SO2CF3)2 or LiC(SO2CF3)3 
high stability to metallic lithium
high decomposition voltage
excellent properties for the formation of the requisite protective films on the electrodes
good solvent power for carbon dioxide: CO2 accelerates the formation of protective films on lithium and LiCn anodes
good solvent power for SO2: SO2 enhances the conductivity over the entire temperature rangexe2x80x94which is particularly important at lower temperaturesxe2x80x94and the formation of protective films on the electrodes.
Another advantage of the invention is that partially fluorinated compounds according to the invention are generally not miscible with water. For the recycling of used batteries, these components can therefore be readily separated from the water-miscible components, i.e., conductive salts and optionally present carbonate solvents, for purification and reuse.
In the above formula (I), the term xe2x80x9cC1-C4 alkylxe2x80x9d should be understood as an unbranched or branched hydrocarbon group with one to four carbon atoms, e.g., a methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 2-methylpropyl or 1,1-dimethylethyl group.
The term xe2x80x9c(C1-C8) alkylxe2x80x9d comprises the groups mentioned under the term xe2x80x9c(C1-C4) alkylxe2x80x9d and, for example, pentyl, 1-methylbutyl, 2-methylbutyl, isopentyl-(3-methylbutyl), 1,2-dimethylpropyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl, 3,3-dimethylpropyl, 1-ethylpropyl, 2-ethylpropyl, n-hexyl, the branched hexyls, particularly, among others, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 3-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-methyl-1-ethylpropyl, 1-ethyl-2-methylpropyl, as well as n-heptyl, n-octyl and the branched heptyl and octyl groups, such as the 1,1,3,3-tetramethylbutyl group;
The term xe2x80x9c(C3-C8) cycloalkylxe2x80x9d comprises the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl group, and, in addition, the term xe2x80x9c(C3-C8) cycloalkylxe2x80x9d also comprises those hydrocarbon groups, which are substituted by one or more of the groups named under xe2x80x9c(C1-C4) alkyl,xe2x80x9d such as 1-methylcyclohexyl, 2-methylcyclohexyl, 3-methylcyclohexyl, 4-methylcyclohexyl, 1,4-dimethylcyclohexyl, 4-tert-butylcyclohexyl and the like.
The term xe2x80x9c(C1-C4) alkyl carbonylxe2x80x9d is to be understood as an unbranched or branched hydrocarbon group bonded via a carbonyl group and containing one to four carbon atoms including the carbonyl carbon atom. Examples of such groups include methyl carbonyl, ethyl carbonyl, propyl carbonyl, 1-methylethyl carbonyl and the like.
The term xe2x80x9c(C1-C8) alkyl carbonylxe2x80x9d comprises the groups cited under the term xe2x80x9c(C1-C4) alkyl carbonylxe2x80x9d and, for instance, the butyl carbonyl, 2-methylpropyl carbonyl, 1,1-dimethylethyl carbonyl, pentyl carbonyl, 1-methylbutyl carbonyl, 2-methylbutyl carbonyl, isopentyl carbonyl (3-methylbutyl carbonyl), 1,2-dimethylpropyl carbonyl, 1,1-dimethylpropyl carbonyl, 2,2-dimethylpropyl carbonyl, 3,3-dimethylpropyl carbonyl, 1-ethylpropyl carbonyl, 2-ethylpropyl carbonyl, n-hexyl carbonyl, 1-methylpentyl carbonyl, 2-methylpentyl carbonyl, 3-methylpentyl carbonyl, 4-methylpentyl carbonyl, 1,1-dimethylbutyl carbonyl, 2,2-dimethylbutyl carbonyl, 3,3-dimethylbutyl carbonyl, 1,2-dimethylbutyl carbonyl, 1,3-dimethylbutyl carbonyl, 2,3-dimethylbutyl carbonyl, 1-ethylbutyl carbonyl, 2-ethylbutyl carbonyl, 3-ethylbutyl carbonyl, 1,1,2-trimethylpropyl carbonyl, 1,2,2-trimethylpropyl carbonyl, 1-methyl-1-ethylpropyl carbonyl, 1-ethyl-2-methylpropyl carbonyl, and n-heptyl carbonyl groups.
The term xe2x80x9c(C3-C8) cycloalkyl carbonylxe2x80x9d comprises cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl groups, which are bonded via a carbonyl group, and, in addition, the term xe2x80x9cC3-C8 cycloalkylxe2x80x9d also comprises those groups, which are substituted by one or more of the groups named under xe2x80x9c(C1-C4) alkyl,xe2x80x9d e.g., 4-methylcyclohexyl carbonyl and the like.
In the context of the invention, partially fluorinated compounds or groups are compounds or groups in which at least one but not all of the carbon-bonded hydrogen atoms of the corresponding compound or the corresponding group are replaced by fluorine.
In the context of the invention, perfluorinated compounds or groups are compounds or groups in which all carbon-bonded hydrogen atoms of the compound or group are replaced by fluorine.
According to the invention, a solution to the safety problem of secondary lithium batteries is achieved by the use of compounds of formula (I) as essential components of the electrolyte system.
In principle, the compounds of general formula (I) are partially fluorinated diesters derived from diols.
The group R1 of the compounds of formula (I) is essentially a fluorinated group. This means that in group R1 at least one hydrogen is replaced by a fluorine atom. Group R1, however, can also be perfluorinated. Particularly advantageous electrolyte systems are obtained, for instance, with compounds of formula (I) in which group R1 is (C1-C4) alkyl, wherein three and as far as possible up to seven hydrogen atoms are replaced by fluorine atoms. Particularly preferably, R1 is CF3, CHF2 or CH2F. Very advantageously, R1 is CF3.
In compounds of formula (I), the group R2 may be un-fluorinated, partially fluorinated or perfluorinated. An electrolyte system with outstanding performance characteristics results, for instance, with a content of one or more compounds of formula (I) in which R2 is (C1-C4) alkyl carbonyl, wherein optionally, as far as possible, up to five hydrogen atoms are replaced by fluorine atoms. Of particular interest are compounds in which R2 is CH3CO or CH3CH2CO in which, as far as possible, up to five hydrogen atoms may be replaced by fluorine.
In yet another advantageous embodiment the electrolyte system contains compounds of formula (I) in which the group R3 is (C1-C4) alkyl. Especially advantageously, the group R3 is a hydrogen atom or a methyl group.
Also of particular interest are systems containing diesters of formula (I) in which m represents 1.
Highly advantageous systems are also obtained if the electrolyte system contains at least one compound of formula (I) in which n is 1 or 2.